Elena is quite correct. I guess I went a little further in my trespass than she did - up to the church, from which you can also look down onto the tiny walled cemetery.
I came across two sets of gates. One set onto and off the range/property and a second set (of two parallel gates) before you go up to the church and finca. The church and finca are about a kilometre or two from the entrance and exit of the property. So, you've got two concentric rings of fencing and gates, one onto the property and another one should you go up to the church (there's yet a third ring around the finca itself).
To get onto the range/property I had to step over the property fence (easy enough, it's been 'lowered' by previous walkers) and to get up to the church I had to roll under two more fences (no problem in dry conditions). I didn't notice key or phone number at either set of gates, but in truth I wasn't looking for them as I was unaware that that might be the procedure.
I hope I haven't made my little interlude sound too dramatic. Just letting walkers/peregrinos know that going through the property (and up to the church) might not be without confrontation or altercation. There's little difference in distance (500m more in fact?) by going through the private property, so going through it implies that you have:
1) an interest in Romanesque churches, this particular one is in a pretty parlous state and there are many other better examples along the way, or
2) you want to make a point about public access.
From the church, it should be a tranquil spot from which to sit and enjoy the view, but of course you can't relax too much when you are blatantly on someone else's property without their permission.
The dogs were not menacing to me - I never saw them, only heard them. They were the ones that spotted my presence and set off the alarm as I was on my way off the property (neither the dogs nor the owners had noticed me up at the church right next to the finca). The dogs came nowhere near me and were well above me (enclosed within the third perimeter around the finca?) but they did alert the owners/factotums of the owners and their barking did set off the cattle stampede.
I had spoken with the hospitaleros in Cuenca and Villaconejos del Trabaque (no mention of a key to me) and later to someone in one of the tourist offices (Cifuentes?) after I had passed through the property, at no stage did any of them say or imply that the property should be avoided but neither did they condone going through it. It is clearly a bone of some contention.
Buen camino, whichever way you choose to go.